The present invention relates to haemodynamic monitoring of human or animal patients. Baxter's Haemodynamic and Oxygenation Parameters lists a large number of body parameters which generally each have normal ranges of values for healthy patients. Departures from these values are useful tools for doctors treating patients to assist in diagnosis of the patient's condition, or for monitoring treatment or progress of a patient.
Such parameters are particularly valuable in intensive care situations, where they can give vital early warning signs that a patient needs attention.
Some of these parameters are very straightforward such as body temperature, mass and volume, heart rate, diastolic and systolic blood pressures, and even blood oxygen saturation. However, there are several which are more complicated and which require sophisticated monitoring systems.
The accompanying drawing is a schematic representation of the mammalian vascular system. The heart 10 has four chambers, the left and right atria (LA, RA) and the left and right ventricles (LV, RV). Each atrium supplies its respective ventricle. The right ventricle pumps blood to the lungs 12 through the pulmonary arteries 14.
Oxygenated blood is returned to the left atrium through the pulmonary veins 16. The left ventricle pumps blood into the aorta 18 which, through connecting arteries 19, supplies most parts of the body 20. Oxygen depleted blood returns through the veins 21 before arriving, finally, at the superior or inferior vena cava 22 which enters the right atrium.
Four basic blood pressures are of interest: Mean Pulmonary Artery Pressure (MPAP) (at 14); Mean Pulmonary Capillary Wedge Pressure (MPCWP or MPWP) also known as Mean Pulmonary Artery Occlusion Pressure (at 16); Central Venous Pressure (CVP) (at 22); and Mean Arterial Pressure (MAP) (at 14). Moreover the actual flow volume is also of significant interest (the cardiac index-CI), from which the Cardiac Output (CO) can be determined. CI is the blood flow per unit area of body surface (in square meters). Cardiac index, along with blood oxygen saturation levels, gives valuable information about a patient's condition, along with other parameters deducible from the foregoing, such as vascular resistance, stroke volume and work, etc.
Mean arterial blood pressure is the easiest to measure as this is what is measured when a person's blood pressure is normally taken. MAP is given by
                    MAP        =                              ASP            +                          2.              ⁢              ADP                                3                            I      where ASP is the Arterial Systolic Pressure and ADP is the Arterial Diastolic Pressure. This formula assumes normal heart operation where, over each heart cycle, one-third is in systole and two-thirds is in diastole. The other three pressures are, however, difficult to measure and hitherto it has been done directly with quite a large degree of interpolation of results and some scepticism as to accuracy.
Currently, a catheter is available which comprises a flexible tube having an inflatable balloon at its tip, together with various pressure and temperature transducers. The catheter is inserted into a substantial vein 21 and fed back towards the heart. After insertion, the balloon is inflated so that it is carried by the blood flow and until it enters the right atrium, passes to the right ventricle and then into the pulmonary artery 16, where the balloon wedges in a first or second division of the pulmonary arteries.
When the balloon is deflated, the transducer measures directly Pulmonary Artery Pressure which, gives MPAP on the basis of a similar calculation to formula I above.
When the balloon is inflated, the pressure on the far side of the balloon from the heart is detected and taken as the MPCWP, or at least indicative thereof in a, hopefully, reproductive way. CVP is taken as an arbitrary proportion of MAP and other parameters, but otherwise is usually directly measured non-invasively, using a CVP catheter.
To measure Cardiac Output a thermo-dilution method is employed by injecting a set amount of a cold liquid into the catheter which exits the catheter upstream of the temperature transducer in the catheter. By monitoring the rise in temperature, the rate of fall, extrapolating the area under the graph, is indicative of the rate of blood flow.
A problem with this arrangement is that firstly it takes considerable skill to insert the catheter in the first place, not to mention the danger to the heart by insertion through it of the catheter. Secondly, there is no constant monitoring of three of the parameters mentioned above, namely MPAP, MPCWP and CI/CO, the first two being alternative measurements and the third being an average of at least three measurements within 10% of each other in a window of time. However, it is feasible that constant monitoring can be arranged but it requires more complication in the design of the catheter and constant known volume rate injection of the cold liquid. In any event, however, there are serious doubts as to the accuracy of measurements made and parameters calculated thereon. Physicians tend only to employ the results generated as a guide to, or confirmation of, diagnoses employing other methods, and the types of treatment that may be required.
An alternative means of measuring Cardiac Output employs a Doppler Oesophogeal probe; but that is only capable of determining Cardiac Output and Index.
Nevertheless, there is a need for a simpler and more reliable means of determining these essential haemodynamic parameters and it is an objective of the present invention to provide such a means.